Wavy waveguide offers bright future for high efficiency solar cells

Scientists show that solar concentrators that don't need to track the sun may …

Solar energy is looking better and better in the light of public reaction to Japan's recent travails. Current technology works, and it works pretty well. But to get vast amounts of power, you need to cover a fair patch of terrain in solar cells. This is because the conversion efficiency of solar cells is not even close to the maximum we'd expect based solely on thermodynamics.

Efficient solar cells are close to a reality, having reached 40 percent efficiency or better. Unfortunately, there is a small fly in the ointment: the cells are really expensive to make, and sometimes involve rare materials. One answer is to make the cell smaller and focus sunlight from a wider area on to it. Solar collectors are not new, but the standard way of making collectors involves moving parts that track the sun. Much better to do it with waveguides.

There have been earlier attempts to use waveguides to make a solar concentrator. Indeed, based on purely thermodynamic considerations, waveguide-based solar concentrators should outperform mirror-based systems by quite a margin. In practice, they don't, due to a fundamental limitation that comes about from the physics used to construct them.

Building a waveguide-based solar concentrator

Waveguide-based solar concentrators make use of two very cool properties of matter. First, when an atom or molecule absorbs a photon of light, it will often emit it again at a lower frequency. So, you put in blue light and you get out green light. This means that we can use atoms to absorb light from the sun, which comes from a wide range of directions over the course of the day, and have it re-emitted under controlled circumstances. This is where our second cool property helps us out.

We think of excited atoms and molecules as emitting light of their own volition, but the direction—and to some extent the color—of the emitted light depends strongly on the environment. This is all due to the fact that light is emitted into modes. Which mode light is emitted into is purely random, provided all the modes have the same number of photons in them.

But the number of modes available is something that we can engineer. For instance, a waveguide can be thought of as having a high density of modes, because the small number of waveguide modes are reinforced by their own interference—the modes look as if they have a lot of photons in them because of the constructive interference caused by photons interfering with themselves. This makes it highly probable that a nearby molecule will emit into the mode of the waveguide.

This preferential emission into the waveguide is exactly the physics that researchers who make waveguide-based concentrators rely on. They place their absorbing molecules on top of a waveguide. Sunlight excites the molecules, which then emit a slightly redder wavelength of light that travels along the waveguide to the solar cell. Essentially, you can imagine making a plate glass window which, instead of transmitting the light that hits it, redirects it all to exit along the edges. This can be a very bright light indeed.

It all sounds good, but it doesn't work. Typically, the light is concentrated by a factor of two to ten, which is close to nothing. Worse than that, the performance gets worse as the illuminating sunlight gets stronger. The reason the concentrator fails is that anything that can emit light can also absorb it. In other words, any of the molecules that are waiting for sunlight to excite them can instead absorb light that is in the waveguide. For pretty fundamental reasons, it seemed that this was never going to be beaten.

Take your principles and shove 'em where the sun don't shine

Remember I said that when an excited molecule wants to emit it has to emit into a mode? Well, the same is true for absorption: a molecule has to absorb light from a mode as well. So what happens if the light mode ceases to exist where the molecules are? Or, more precisely, the light in that mode must have zero intensity? Simple, no absorption.

This is exactly what the researchers have done. The waveguide is exactly the same: a strip of high refractive index material with a thin coating of a low refractive index material. The absorbing molecules sit in a layer on top of the low refractive index material.

To change the way that the modes behave, the researchers change the thickness of the absorber material so that it looks a bit like a repeating staircase. This thickness changes the behavior of the light in the waveguides. Essentially, the steps change where the high intensity points in the waveguide modes fall. This ensures that each thickness emits into modes that have low intensity in the remaining areas.

The matching of molecules to particular modes of the waveguide ensures that absorption of the guided light is minimized, and allows much higher concentration of light to be reached. Or at least that is the idea. The researchers showed that they could increase the transmission efficiency of waveguides based on these structures by some 20 percent. However, they did not actually show much in the way of improvement in solar concentration.

However, part of this is due to the size of their test sample. Their simulation results really shine, estimating that, for very large structures with geometric concentrations better than 100, the structured waveguides really start to win. However, the ultimate performance, even simulated, is not that spectacular, because the actual concentration factor (that is, illumination intensity out divided by illumination intensity in) is only about 25.

No doubt this will be improved by optimizing the structure of the absorber layer. But as the authors point out, there are limits to that, because the waveguide mode structure and the patterns will eventually repeat, leading to unwanted absorption. It seems likely that this will find use in fields like space applications where you really need solar cells, but can't afford the space and weight of inefficient cells.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku

I guess the picture at the top is a waveguide. Idiot question time: From which direction is the sun's energy coming? Also, are these little red dots solar cells? One on the left and three on the right?

Also: "looks a bit like a repeating staircase"

No jargon there but you still lost me ... Any chance of a linky to a graphic that shows what you mean?

"This is because the conversion efficiency of solar cells is not even close to the maximum we'd expect based solely on thermodynamics. Efficient solar cells are close to a reality, having reached 40 percent efficiency or better."

The Shockley–Queisser limit is less than 40 percent for a single-junction cell, which are those that are generally commercially available as you refer to. And commercial cells are not that far off -- I think SunPower is in the upper-20's at this point. I know the article is in the context of solar concentrators for which the limit is much higher because the thermodynamic considerations are different, but I'd thought I'd point this out for others.

I've been reading about these solar efficiency improvements for decades. None of them ever scale up.

Nothing wrong with research though.Its when nitwit politicians get paid off to push stupid ideas about economies of scale on tech that doesn't scale, wasting $hundreds of billions of Global Warming fighting dollars on deployment of not ready for prime time solar tech that the damage really occurs.

Today's solar costs an average of 70 cents a kwh when 85% gas backup and 7 times sized transmission lines are included and another buck a kwh when green storage replaces the deadly filthy GHG spewing gas backup.

Not one of said nitwits has answered the question of how a solar powered civilization survives another Tambora volcano which in the early eighteenth century pretty well clouded the earth over for two years.

Let's assume that this breakthrough here works out to be deployable at the same cost per sq foot as current solar. Now we are down to 35 cents a kwh with gas backup and 85 cents with green storage.

What is the point of this idiocy?

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

While I do like the idea of improving efficiency of solar cells, this is just research that might, possibly, in a decade or so (assuming no dead-ends on this particular facet of efficiency research) could be available commercially, maybe even to consumers.

However, I would like to see coverage on stuff that is much, much closer to ready for prime time, Ars. Seriously. This is neat and all, but a lot of the stuff I've seen and read about research into things that could make our lives better is always '5, 10 years away' from actually doing so. And we hear about a thing once, and then it fades into the background, more or less. Nothing is heard from them afterwards, at least in the mainstream (which I class Ars Technica as, personally). Obviously if I were visiting more science-oriented sites, I would hear more, though.

Back to this specific article, I would love to know how that picture is supposed to be annotated, if at all.

I've been reading about these solar efficiency improvements for decades. None of them ever scale up.

Nothing wrong with research though.Its when nitwit politicians get paid off to push stupid ideas about economies of scale on tech that doesn't scale, wasting $hundreds of billions of Global Warming fighting dollars on deployment of not ready for prime time solar tech that the damage really occurs.

Today's solar costs an average of 70 cents a kwh when 85% gas backup and 7 times sized transmission lines are included and another buck a kwh when green storage replaces the deadly filthy GHG spewing gas backup.

Not one of said nitwits has answered the question of how a solar powered civilization survives another Tambora volcano which in the early eighteenth century pretty well clouded the earth over for two years.

Let's assume that this breakthrough here works out to be deployable at the same cost per sq foot as current solar. Now we are down to 35 cents a kwh with gas backup and 85 cents with green storage.

What is the point of this idiocy?

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

1. In the 25 years I've been watching, we have gone from 11% efficency for commercial panels to 25%+. The efficiencies are coming.2. Australia has reached parity with coal fired power-stations; with a carbon tax the solar industry here is going to be huge.

I want nuclear but my nation's s/stupid/bought/old chums/ leaders won't talk about it. There really is no reason for us to not use both/multiple sources but bean counters don't like having to count past 1.

While I do like the idea of improving efficiency of solar cells, this is just research that might, possibly, in a decade or so (assuming no dead-ends on this particular facet of efficiency research) could be available commercially, maybe even to consumers.

However, I would like to see coverage on stuff that is much, much closer to ready for prime time, Ars. Seriously. This is neat and all, but a lot of the stuff I've seen and read about research into things that could make our lives better is always '5, 10 years away' from actually doing so. And we hear about a thing once, and then it fades into the background, more or less. Nothing is heard from them afterwards, at least in the mainstream (which I class Ars Technica as, personally). Obviously if I were visiting more science-oriented sites, I would hear more, though.

Back to this specific article, I would love to know how that picture is supposed to be annotated, if at all.

One of the ways to get it to commercial success is to have multiple people/competitors/etc working on it. So many great technologies have not gone anywhere because nobody but the inventor or lab were the only ones that knew about it. I'm thinking specifically of the 1948 actress that developed the technology for jumping frequencies that only became widespread when wifi began to be created. Or the Amiga. There are more examples but I can't be bothered tracking them down.

As best I can tell, the picture shows a pointsource of light on the left starting in a thin, low-refractive material then passes into a higher-refractive material where it begins to spread. Light tends to refract outward, so the picture only makes sense to me if the light moves from left to right. From there, I see an interference pattern resulting in 3 concentrated beams of light. Whereas the incoming light is red surrounded by green surrounded by blue, the outgoing light is just green surrounded by blue. This indicates the incoming light is primarily red and the outgoing is primarily green. These are not necessisarily real red and green, but correspond to some set of frequencies. However, something doesn't make sense to me. If there were a second point source, say above the first one, I would expect the three outgoing beams to also shift upward. If incoming light is spread all along the left surface, then I would expect outgoing light to also spread along the right surface. So I don't see how it is concentrating light unless something about the waveguide channels any incoming light to those 3 points we see on the right. Thoughts?

@ Sethdayal: While I generally agree that nuclear is better than solar for fighting climate change on a budget, we still need fundamental solar research because of satellites. In space, any vehicle not traveling past Mars uses solar. Small solar efficiency gains can mean more capability. As a comparison, the iPad 2 and the iPhone 4S have the same A5 processor clocked at 1GHz and 800MHz respectively because the iPad 2 has a bigger battery and can power a faster processor. So a bigger energy budget = more capability per weight/size/cost. The same is true for satellites. Communications, imagery (like Google Earth), and GPS all benefit from advances in solar. In addition to satellites, solar works very well for off-grid infrastructure in mid-lattitude deserts like the Middle East, northern Africa, much of the Americas, and the Australian interior.

If this tech works, then they still need to find the optimal configurtion, scale it up, and mass produce it. Each step is probably several years worth of work.

I've been reading about these solar efficiency improvements for decades. None of them ever scale up.

Nothing wrong with research though.Its when nitwit politicians get paid off to push stupid ideas about economies of scale on tech that doesn't scale, wasting $hundreds of billions of Global Warming fighting dollars on deployment of not ready for prime time solar tech that the damage really occurs.

Today's solar costs an average of 70 cents a kwh when 85% gas backup and 7 times sized transmission lines are included and another buck a kwh when green storage replaces the deadly filthy GHG spewing gas backup.

Not one of said nitwits has answered the question of how a solar powered civilization survives another Tambora volcano which in the early eighteenth century pretty well clouded the earth over for two years.

Let's assume that this breakthrough here works out to be deployable at the same cost per sq foot as current solar. Now we are down to 35 cents a kwh with gas backup and 85 cents with green storage.

What is the point of this idiocy?

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

I never thought about what would happen if the sunlight got blocked! =P After seeing what has happened with Nuclear with Japan and Chernobyl before it, I'm leery about Nuclear. But, I've read that Thorium reactors are much more efficient and require power to keep the reaction going meaning you can't have a runaway chain reaction and meltdown. If you pulled the plug the reactor would just stop.

In any event, what's needed is not particularly a new power source--there are plenty if we get creative. What we need is an efficient way to store the energy. The reason why we rely on fuels is because batteries can't store the energy with enough density or discharge it quickly enough. Put together a room temperature super conducting capacitor and you solve that problem.

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

I can see it used in specialty applications, where you have to get maximum output from a fixed area, but a 25-to-1 concentrator isn't going to take any market share unless it is dirt cheap. File it in the "curious and vaguely useful" file.

You also have the issues of remote locations. Often times it just is not feasible to run power lines to remote locations to power them. RTGs are a heck of a lot more expensive per watt than solar and have a host of issues, and generators, though cheap, have to have fuel delivered to their remote location which jacks up the cost significantly on top of having to use a consumable to generate power.

In a residential in situ application solar is actually pretty decent, at least when you take out the profit and overhead of what a utility charges you for your electric. Current costs are roughly $3 per watt for residential solar. If you include grid-tie converter and labor it is more like $4-4.50 per watt and off-grid w/ battery backup in the $5 range per watt. Costs are somewhat lower on really big deployed projects because you can bulk source the panels and DIY installation even lower (I priced a system for my hypothetical future home at around $3.30 per watt for a 3500w system DIY grid tie system w/ no battery backup not inclusive of tax rebates).

Even if you exclude tax rebates it can pay itself back over time. Most of the US averages about 4-5hrs of insolation per day, which works out to 1400-1900w/hrs per year per installed watt of solar panel.

At electrical rates (including service/transmision) at about 14 cents per kw/hr, that is about 20-25 cents per year of electrical generation per watt of installed power. A long time for "pay back" tipping point, but still only roughly 15 years for a DIY system on the grid and maybe a bit closer to 25 years for a battery backup/off grid system that you are paying to have installed. Again not inclusive of tax rebates/discounts.

On the cheaper system that is about a 6.7% rate of return, and on the more expensive one a 4% rate of return on your original investment if looking at it from a fiscal point of view. Tax rebates make it even more attractive, but since we are considering this from a gov't shouldn't be supporting it point of view, lets just ignore.

Even better, not energy generation, we should just mandate/encourage things like solar hot water heating. Especially in areas of the country that get even more insolation, but even in poor insolation areas, you can drastically reduce your energy footprint for low cost. I think I figured something like a 6-7 year payback on a solar hot water heating system for my current townhouse (I am not going to be here long enough to take advantage) assuming I use somewhere roughly around 30% of my electric bill on hot water and that 80% of my hot water could come from the solar hot water heating system.

I've been reading about these solar efficiency improvements for decades. None of them ever scale up.

Nothing wrong with research though.Its when nitwit politicians get paid off to push stupid ideas about economies of scale on tech that doesn't scale, wasting $hundreds of billions of Global Warming fighting dollars on deployment of not ready for prime time solar tech that the damage really occurs.

Today's solar costs an average of 70 cents a kwh when 85% gas backup and 7 times sized transmission lines are included and another buck a kwh when green storage replaces the deadly filthy GHG spewing gas backup.

Not one of said nitwits has answered the question of how a solar powered civilization survives another Tambora volcano which in the early eighteenth century pretty well clouded the earth over for two years.

Let's assume that this breakthrough here works out to be deployable at the same cost per sq foot as current solar. Now we are down to 35 cents a kwh with gas backup and 85 cents with green storage.

What is the point of this idiocy?

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

I never thought about what would happen if the sunlight got blocked! =P After seeing what has happened with Nuclear with Japan and Chernobyl before it, I'm leery about Nuclear. But, I've read that Thorium reactors are much more efficient and require power to keep the reaction going meaning you can't have a runaway chain reaction and meltdown. If you pulled the plug the reactor would just stop.

In any event, what's needed is not particularly a new power source--there are plenty if we get creative. What we need is an efficient way to store the energy. The reason why we rely on fuels is because batteries can't store the energy with enough density or discharge it quickly enough. Put together a room temperature super conducting capacitor and you solve that problem.

Chernobyl said more about the nature of the Soviet Union than it did about nuclear power. Plants of that type were rejected almost immediately when the first civilian designs were proposed. The closest we came to that design was in submarines and some of the earliest experimental test sites.

Considering the facility in Japan was forty years old and went through one of the most devastating natural disasters ever witnessed by humans, it did pretty damn well in spite of all the bad choices and other issues. Newer plant designs would handle damage far better, with shutdown occurring pretty much automatically if everything is not in order and without complex software monitoring.

A recent episode of Nova went into some detail about the Japan plant. The seawall actually would have worked if the tsunami were triggered by a mid-ocean quake but as the epicenter was so near the enter region dropped a significant distance. The wall was in intact but a couple feet lower than when it was built. The extremity of this event was such that the way that plant held up was pretty remarkable. And we know how to do it much better today.

I've been reading about these solar efficiency improvements for decades. None of them ever scale up.

Nothing wrong with research though.Its when nitwit politicians get paid off to push stupid ideas about economies of scale on tech that doesn't scale, wasting $hundreds of billions of Global Warming fighting dollars on deployment of not ready for prime time solar tech that the damage really occurs.

Today's solar costs an average of 70 cents a kwh when 85% gas backup and 7 times sized transmission lines are included and another buck a kwh when green storage replaces the deadly filthy GHG spewing gas backup.

Not one of said nitwits has answered the question of how a solar powered civilization survives another Tambora volcano which in the early eighteenth century pretty well clouded the earth over for two years.

Let's assume that this breakthrough here works out to be deployable at the same cost per sq foot as current solar. Now we are down to 35 cents a kwh with gas backup and 85 cents with green storage.

What is the point of this idiocy?

We already have 3 cent a kwh 24/7 clean and green zero environmental footprint zero GHG Gen III nukes being deployed and ready for deployment on a massive scale. We have MSR and IFR reactors designed and ready to build at utility scale.

We don' t need the solar. What we need is a law that stops Big OIl from buying our leaders so we can get on with a fossil to nuclear conversion.

I never thought about what would happen if the sunlight got blocked! =P After seeing what has happened with Nuclear with Japan and Chernobyl before it, I'm leery about Nuclear. But, I've read that Thorium reactors are much more efficient and require power to keep the reaction going meaning you can't have a runaway chain reaction and meltdown. If you pulled the plug the reactor would just stop.

In any event, what's needed is not particularly a new power source--there are plenty if we get creative. What we need is an efficient way to store the energy. The reason why we rely on fuels is because batteries can't store the energy with enough density or discharge it quickly enough. Put together a room temperature super conducting capacitor and you solve that problem.

That plant was 40 years old, its design even older. That means it came online in the early 1970s.

sethdayal's point about the existing sources of (especially nuclear) power ignores one of the most brilliant political moves from the late forties and fifties as The Friendly Atom was born to support bomb production. The benefits are trumpeted (anybody else remember "Too Cheap To Meter"?) and the costs are externalized. That's what the Price-Anderson Act was all about (Google it.) When it all goes toes-up at the plant, whether from a natural or man-made disaster, we taxpayers (through our government) foot the bill. Now we would have to anyway, but there is a fundamental dishonesty in the way the accounting is figured. The overall life-cycle costs of fission power includes storage and eventual 'disposal' of the fission products, which at this point, are still sitting in water pools at each plant, and the NIMBY resistance of just about everybody whose neighborhood is proposed to be used (Google [Yucca Mountain]), OR through which it might travel. We may have to put the (still-hot) 'waste' (in drums that can survive the undersea environment for long enough) into the subduction zones in the oceans, where it will be (eventually) returned to the mantle. Everything gets recycled sooner or later, absolutely everything (whether just on Earth, or our Solar System, or at larger scales). The trick is not to get poisoned by it, or screw-up our food supply with it, before it gets back into the cycle.Oh yes, also; as for a couple of years of volcano dimming, we can get around that by using a mix of wind power with the sun, and wind power is still becoming better at several scales, and is already more economical than coal (not to mention entirely clean).